There are a variety of apparatuses for determining and measuring magnetic fields. These are, among others, Hall sensors, anisotropic magnetoresistance (AMR) sensors, field plates, field coils, fluxgate sensors, giant magnetoresistance (GMR) sensors, tunnel magnetoresistance (TMR) sensors, and superconducting quantum interference devices (SQUIDs).
One very simple principle, based on a coil and a soft magnetic core, is the fluxgate sensor. The measurement principle is based on remagnetization of a soft magnetic core with the aid of an exciter coil, and detection, using a measurement coil (pickup coil), of the time-dependent flux thereby generated. The change in flux is determined by the magnetization curve of the soft magnetic core as a function of the external field. The more quickly the remagnetization occurs, the greater the voltage in the pickup coil. The voltage elevation can be generated both by a steeper magnetization hysteresis (greater permeability) and by an increase in the frequency of the exciter coil.
An evaluation method that is often used is measurement of the second harmonic component of the signal in the pickup coil. This component corresponds substantially to the nonlinear portion of the transfer function that is attributable to saturation of the core. The amplitude of the second harmonic component is proportional to the external field.
One such method is described, for example, in Drljaca, P. M. et al., “Low-Power 2-D Fully Integrated CMOS Fluxgate Magnetometer,” IEEE Sensor Journal, Vol. 5, Issue 5, pp. 909-915 (2005). The dimensions of the fluxgate cores here are: length 1400 μm, width 20 μm, and thickness 7 μm. In this method, the signal shape is broken down into its Fourier components in order to separate the second harmonic component dependent on the external field. It is important here that the amplitude of the second harmonic be appreciably greater than the system noise. This amplitude decreases with the volume of core material, however, and the possibility for miniaturization is therefore limited.
Another measurement method known in the literature (Walter Heinecke, PhD thesis: “Measurement of magnetic fields and field differences using saturation core probes with the direct time coding method,” Braunschweig, 1975) is to measure the point in time of remagnetization on the basis of the voltage swing of the voltage induced in a pickup coil. This point in time depends on the external field and is thus an indication of the field to be measured.
For accurate measurement of this point in time, it is necessary that the voltage pulse resulting from remagnetization have a rise time that is as steep as possible. An increase in frequency is not helpful, since the rise time then becomes narrower, but the offset width becomes less to the same extent, so that no improvement in resolution can be achieved. All that remains is to make the magnetization hysteresis as steep as possible by appropriately selecting the material and the manufacturing process.
For miniaturized fluxgate sensors there are limits to the optimization of the material and process because the boundary conditions of the MEMS manufacturing process allow little variation. In addition, because of the demagnetization factor, short fluxgate cores exhibit flatter hysteresis curves than non-miniaturized fluxgates of the same material. The miniaturization of the coils and of the coil core also causes the signal at the pickup coils to increasingly small (for the same magnetization hysteresis), so that evaluation becomes increasingly difficult.